Chuck Schumer is quietly letting out the word: He’s far from sold that Chuck Hagel will be a staunch advocate of Israel.

And it’s his vote that counts.

Schumer, the most powerful Jewish Democrat in Congress, has been noncommittal in his public statements on Hagel’s nomination. But privately, several sources say he has told senators it would be “very hard” for him to support Hagel as the next defense secretary because of his positions on Israel over the years. In New York, Schumer has told allies and power brokers in the Jewish community that he’s uneasy about Hagel’s nomination, a concern he reiterated at a private breakfast in Manhattan’s posh Park Avenue Winter restaurant on Wednesday.

If Schumer were to oppose Hagel, it would almost certainly amount to a fatal blow to his candidacy since a number of pro-Israel Democrats who are squeamish about the nominee could very well be influenced by the No. 3 Democrat’s position. It would also give bipartisan political cover to Republicans and neocons fighting Hagel’s nomination.

Still, Schumer could also provide critical support for Hagel’s nomination. Should he support Hagel, it very likely would ride on what the former Nebraska GOP senator eventually says on Israel at an upcoming one-on-one meeting with the New York Democrat and during his confirmation hearings before the Senate Armed Services Committee. Schumer declined to be interviewed Thursday for this story. The White House also declined to comment.

With the retirement of Connecticut Sen. Joe Lieberman, Schumer has emerged as perhaps the most hawkish Jewish member of the Senate Democratic Caucus with regard to Israel, a subject that has caused rifts between President Barack Obama and pro-Israeli activists throughout his first term. The Hagel nomination has only raised the temperature on their relationship, and if Hagel fails to placate concerns from the pro-Israeli groups, Schumer will be in a tough spot between his Jewish base and the administration pushing hard for his confirmation.

Schumer’s relationship with Obama over Israel has been rocky as well, with one source close to the White House expressing frustration over the New York Democrat’s cool response to Hagel so far. In the past, Schumer has been harshly critical of the president’s handling of the peace process between the Israelis and Palestinians.

But he has also come to the defense of the Obama administration in key times, including over the Iranian threat during the presidential campaign, when Mitt Romney was aggressively courting Jewish voters.

In a previously unreported episode, Schumer told Israeli Prime Minister Benjamin Netanyahu that he was betting on the wrong candidate after he bashed the Obama administration over Iran in the heat of last fall’s presidential race, according to a person familiar with the telephone call. Schumer’s implicit message to his friend Netanyahu: Continuing to attack Obama will only undercut their efforts to win strong support from the administration in the president’s second term.

Abe Foxman, head of the Anti-Defamation League, is not opposing Hagel but has been lukewarm about the nominee. The influential group is pushing its allies in the Senate to pin the former Nebraska senator down on his views on Israel.

“It’s a troubling gestalt,” Foxman said in an interview of Hagel’s past comments. “He doesn’t have to wait for hearings and things like that either. … He’s so out of step with a lot of the president’s position [on certain issues. He should] explain that, too.”

Hagel has been critical of Israel’s dealings with Hezbollah in Lebanon, including during their 2006 military conflict. In 2006, Hagel declined to sign a letter calling on the European Union to list Hezbollah as a terrorist organization. After leaving office, Hagel signed onto a 2009 letter urging Obama to begin peace talks with Hamas. And he’s still paying the political price for comments he made in 2006, when he referred to pro-Israel forces as the “Jewish lobby” that tends to “intimidate” lawmakers.

On Iran, critics point to Hagel’s votes against a resolution calling the Iranian Revolutionary Guard Corps a terrorist organization as well his past opposition to sanctions against the country.

Recognizing his vulnerability on the issue, Hagel has already tried to rebut the criticisms that he has a hostile record toward Israel, telling his hometown paper, the Lincoln Journal Star, that he has demonstrated “unequivocal” support for the country.

Hagel added that there’s “not one shred of evidence that I’m anti-Israeli, not one [Senate] vote that matters that hurt Israel. … I didn’t sign on to certain resolutions and letters because they were counter-productive and didn’t solve a problem.”

Some Jewish groups on the left — such as the National Jewish Democratic Council — have been more open to Hagel since they say it is the president who will set the direction of the administration on Israel, not the Cabinet secretary.

But those assurances alone are not yet enough to satisfy Schumer or others in the Jewish community far more concerned about his nomination. By holding out his support, Schumer can extract from Hagel certain concessions over Israel.

Still, others are more skeptical that Schumer’s efforts to appease the Jewish community will pay dividends.

“Schumer will tell everyone he’s ‘working behind the scenes,’” Ed Koch, the vocal former mayor of New York City, scoffed in an interview with POLITICO this week.

Some pro-Israel groups — like the American Israel Public Affairs Committee — have taken a hands-off approach to the Hagel nomination so far.

But several sources in the New York and Washington Jewish communities said that despite the stand-down from some of the more vocal pro-Israel groups, there is going to be growing pressure on a key group of senators on the committee involved in the Hagel proceedings — along with Schumer, who is not on the Armed Services Committee but is seen as a crucial bellwether — to ensure specific answers on policy, as well as Hagel’s past statements, during the hearings and in the lead-up to them.

One Jewish leader said there is fear Schumer will not make a clear stand on Hagel. If this happens, it could present an opportunity for Democratic New York Sen. Kirsten Gillibrand, who sits on the Armed Services Committee but has typically been in Schumer’s shadow in the state’s political system, to distinguish herself on the issue.

“She has an opening to do something if he doesn’t,” said the source.

But another prominent New York Jewish figure, referring to an often-quoted local Orthodox Jewish assemblyman, said people on the left expect Schumer to ultimately back Hagel, in part because of his own future ambitions.

“He has to decide whether he wants to be Dov Hikind or whether he wants to be a leader.”

Not every Jewish senator is skeptical of Hagel. Senate Armed Services Committee Chairman Carl Levin (D-Mich.) and Senate Intelligence Committee Chairwoman Dianne Feinstein (D-Calif.) have signaled they’d support his nomination. Others, like Sens. Ben Cardin of Maryland, Frank Lautenberg of New Jersey, Richard Blumenthal of Connecticut and Barbara Boxer of California, have withheld their support so far.

Now with the Hagel nomination on the Senate’s front burner, the relationship between Democratic leaders and the White House will be on full display. When news broke over the weekend that Hagel was poised to get the nomination, senior Democrats made clear that the White House would have to work to unite their conference — namely over Hagel’s positions over Israel. And Republicans made clear that Israel would be a flash point in the confirmation proceedings.

“If there’s one issue that will almost always get unanimous support, or pretty darn close, it’s support for Israel,” said Sen. Mike Johanns, a fellow Nebraska Republican and longtime friend of Hagel’s. “He needs to explain what his own words mean.”